1. Field of the Invention
The present invention relates to a photodiode made from a P type HgCdTe semiconductor substrate in which an N doped zone is formed on one face thus providing a PN junction and having metallizations giving access to the P and N zones.
Such photodiodes are sensitive to the near infrared and the wavelength on which their maximum sensitivity is centered depends on the composition of the semiconductor crystal of the substrate. This composition is defined by the molar fraction x of the formula Hg.sub.1-x Cd.sub.x Te, which molar fraction may vary form 0 to 1. These photodiodes are used particularly in optical fiber telecommunications, at wavelengths of 1.30 and 1.55 microns for example.
2. Description of the Prior Art
Photodiodes of the above defined type are already known which however have the drawback of a relatively slow response, i.e. they transmit badly the high components of the frequency spectrum of the signals which modulate the light radiation which they detect. At the present time, the cut-off frequencies of these photodiodes are of the order of 500 to 800 MHz. It is obvious that this limits the flow of information which may be transmitted through optical fiber telecommunications systems. It is therefore essential to have photodiodes with higher cut-off frequencies. Now, it is known that the cut-off frequency of a photodiode is all the higher the lower the product RC of the series resistance R and of the capacity C of its equivalent diagram. Thus, one means for increasing the frequency consists in reducing the value of the series of resistance R of the equivalent diagram.
Now, this resistance R is the total series resistance of the photodiode, and the two main components of this resistance are the resistance of the P type HgCdTe semiconductor substrate and the resistance of the contact between the substrate and the metallization for access thereto.
To reduce the resistance of the substrate, the thickness thereof may be reduced by mechanical polishing, for example to 50 microns, which represents substantially the lower limit of what it is possible to do. Then, for example in the case of a P type HgCdTe disk with a resistivity of 10 ohm.cm and a diameter of 150 micron, a resistance of the substrate is obtained of the order of 200 ohms.
However, in order to obtain the total series resistance R of the photodiode, the value of the contact resistance with the metallization should be added thereto. Now, it is difficult to reduce the contact resistance with known techniques which consist in inserting, between the P type semiconductor crystal and the metal of the metallization, an overdoped P.sup.+ layer. In fact, as is known, the alloy HgCdTe is difficult to overdope, and there always exists a rectifying effect between the P type substrate and the P.sup.+ layer, which increases the resistance of the contact.
The present invention overcomes this drawback by providing a photodiode whose contact resistance between the substrate and the metallization is low, so as to obtain a low total series resistance, and so a high speed response.